Wiping hot dipped galvanized wire or strip

ABSTRACT

The disclosure relates to the wiping of hot dipped galvanized wire or strip by drawing it upwardly from a bath of molten zinc through a wiping bed of discrete material which floats thereon. The bed is laterally confined and is continuously or intermittently vibrated to thereby cause it to become more dense and uniform so that the zinc coatings formed on the wire or strip are more uniform in thickness. The bed may be loaded at the top so that a downward pressure is exerted thereon. The disclosure further provides for the vibration of the bed under conditions such that the bed forming material is discharged at a restricted rate from the bottom of the bed while the latter remains substantially uniform and dense, the material thus discharged being replaced by the addition of fresh material to the top of the bed (whereby the bed is self-cleaning). The disclosure may be used in combination with &#39;&#39;&#39;&#39;gas wiping&#39;&#39;&#39;&#39; according to which air is excluded from the wiping bed by continuously passing upwardly therethrough a non-oxidizing gas containing at least a small proportion of hydrogen sulphide or other suitable sulphide forming gas.

United States Patent 1191 Sciffer 1 July 1,1975

i 1 WIPING HOT DIPPED GALVANIZED WIRE OR STRIP [75] Inventor: Jack Pryor Scifier, New Lambton,

Australia [73] Assignee: Australian Wire Industries Proprietary Limited, Victoria, Australia 22 Filed: Mar. 16, 1973 21 App]. No.: 341,851

[52] US. Cl. 427/367; 118/123; 427/57 [51] Int. Cl BOSc ll/lO; C23c 1/00 [58] Field of Search..... 117/102 M, 102 RL, 114 A,

ll7/D1G. l, 119; 118/123, 124

[56] References Cited UNITED STATES PATENTS 3,513,018 5/1970 Taylor ll7/ll4A 3,738,861 6/1973 Sciffer et a1. 117/114 A Primary Examiner-Mayer Weinblatt Assistant Examiner-Edith L. Rollins Attorney, Agent, or FirmCushman, Darby & Cushman 5 7 ABSTRACT The disclosure relates to the wiping of hot dipped galvanized wire or strip by drawing it upwardly from a bath of molten zinc through a wiping bed of discrete material which floats thereon. The bed is laterally confined and is continuously or intermittently vibrated to thereby cause it to become more dense and uniform so that the zinc coatings formed on the wire or strip are more uniform in thickness. The bed may be loaded at the top so that a downward pressure is exerted thereon. The disclosure further provides for the vibration of the bed under conditions such that the bed forming material is discharged at a restricted rate from the bottom of the bed while the latter remains substantially uniform and dense, the material thus discharged being replaced by the addition of fresh material to the top of the bed (whereby the bed is selfcleaning). The disclosure may be used in combination with gas wiping" according to which air is excluded from the wiping bed by continuously passing upwardly therethrough a non-oxidizing gas containing at least a small proportion of hydrogen sulphide or other suitable sulphide forming gas.

9 Claims, 4 Drawing Figures SHEEI w Ln mzomui wScisz FREGUENCY HERTZ WIPING HOT DIPPED GALVANIZED WIRE OR STRIP This invention relates to the operation of wiping hot dipped galvanized, wire or strip.

For many years, it has been a common practice to form zinc coatings on wire or strip by drawing it upwardly from a bath of molten zinc through a wiping bed of oiled crushed charcoal which floats thereon, the zinc coating then being solidified above the bed by cooling in air and/or by a water quench. Various other discrete wiping materials, including sand, vermiculite and coke have been used to some extent in the past but charcoal has been found to be generally superior and for many years has been the wiping material used almost invari ably.

However, the use of a conventional charcoal wiping bed has a number of disadvantages, one of which is that coatings of acceptable quality, can be produced only at relatively slow throughput speeds with the consequence that only a limited degree of control can be exercised over the weight of the zinc coating applied to the wire or strip,

In the specification of our prior U.S.A. Pat. Application Ser. No. 805,145, now US. Pat. No, 3,738,236] we have described an improved method, which permits of the wiping of hot dipped galvanised wire or strip to produce at substantially higher throughput speeds, zinc coatings of similar weight and generally superior quality, to those produced by using a conventional charcoal wiping bed. Briefly, the specification discloses the use of wiping beds formed of hard, inert and free tumbling bodies, and especially of washed and graded river gravel, from which air is excluded by continuously passing upward through the bed, a suitable nonoxidising gas containing at least a small proportion of hydrogen sulphide.

The operation of wiping hot dipped galvanised wire or strip by drawing it through a wiping bed of discrete material from which air is excluded by passing, upwardly therethrough, a non-oxidising gas essentially containing at least a small proportion of hydrogen sulphide or other suitable sulphide forming gas, is hereinafter termed gas wiping.

For different purposes, galvanised wires having zinc coatings of different standards are required, each standard being usually specified in terms of minimum weight of zinc per unit area of uncoated wire surface, e.g. in grams per square metre. There are several generally recognised specifications for different standards of such coatings amongst which a convenient and representative example is, Table 7-24 entitled Minimum ounces of zinc per square foot of uncoated wire surface" of the American Iron and Steel Institute (AlSl) Steel Products Manual: Wire and Rods Carbon Steel. In this table, the type of coating most commonly produced and required in practice is designated Type A, which is the coating weight generally, if not invariably, produced by using the conventional charcoal wiping method, A heavier type of coating, designated Type B, has twice the weight of zinc as Type A, while a still heavier coating designated Type C has three times the weight of zinc as Type A.

The said AlSl specification also makes provision for coatings of a lesser weight than Type A, but these, and also the aforesaid Type B and Type C coatings, have normally been produced by employing different tech niques not involving the use of wiping beds of discrete material.

In order to economise in the use of zinc, the specified minimum coating weight should be exceeded by as lit tle as possible irrespective of the type of coating that is being produced.

It is well known that the speed of the wire through the bed is the principal factor in determining the weight of the zinc coating although it has been found that the influence of speed is markedly less when the aforesaid gas wiping process is used than in the conventional charcoal wiping process. For both processes, however, increases in speed cause a heavier coating to be carried by a wire emerging from the zinc bath and entering the wiping bed. Conventional charcoal wiping however, does not normally permit of the formation of good quality coatings at speeds much above those required for the formation of Type A coatings.

In order therefore to produce coatings of different specified weights using a wiping bed of discrete material, we have endeavoured to identify and utilise other factors which influence average coating weight and, perhaps even more importantly, which would permit of a reduction in the variations in coating weight that are generally found to occur in practice along the length of each wire, and on corresponding portions of similar wires that pass side-by-side through the same wiping bed. Thus, if more uniform coatings could be formed, the average coating weight could be reduced without any individual test results falling below a specified minimum. Since zinc is relatively costly, a reduction in average coating weight would result in significant savings in zinc consumption and hence in cost.

Our investigations have shown that another factor, and the one relevant to this invention, for controlling coating weight is the pressure, hereinafter termed wiping pressure, exerted by the wiping bed on the coated wire as it emerges from the Zinc bath, as an increase in this wiping pressure causes a reduction in coating weight and vice versa.

Consequently, the wiping pressure may be varied by using a discrete material with a different specific gravity to form the wiping bed. Thus, if heavier coatings are required at a constant drawing speed, a material of lower specific gravity may be used and vice versa. Like wise, by using a material of higher specific gravity, e.g. gravel, coatings of the same average weight as those formed in a conventional charcoal bed, may be produced at increased drawing speeds, when gas wiping is used, as disclosed in our aforesaid prior specification.

Also, the wiping pressure may be varied by changing the depth of the bed. In our experience, however, variations in the depth of the bed have in the past exercised only a limited degree of control because of the effects of internal friction between the granules themselves and/or between the granules and the walls ofthe chamber or box containing the wiping bed. As the depth of the bed is increased, for a box of given dimensions, the wiping pressure is found not to increase correspondingly and, perhaps more significantly, to become erratic. We believe these frictional effects, tending to cause the granules to bridge or bind, to be responsible for preventing the desired uniformity and density in the arrangement and packing of the bed-forming granules, and thus to contribute to the variations in coating weight that are found to occur in practice under apparently similar conditionsv It is therefore a general object of this invention to provide improved methods of and means for controlling the magnitude of, and variations in, the wiping pressure of wiping beds used in hot dip metal coating operations, especially galvanising.

Accordingly, the invention provides the method of wiping hot dipped galvanized wire or strip by drawing it upwardly from the bath of molten zinc through a laterally confined bed of discrete material which floats thereon and which is composed of gravel or other hard and inert material in the form of free tumbling bodies or granules characterized in that the bed is continuously or intermittently vibrated thereby to maintain it in a dense and uniform condition so that the coatings formed on the wire or strip are more uniform in thickness.

in addition, the bed may be pressed downwardly by weights; supported on its upper surface or by a pressure plate actuated by springs or by fluid pressure.

Preferably the invention is used in combination with the aforesaid gas wiping procedure.

According to the preferred form of the invention. the bed is confined within a resiliently supported box or other enclosure which is vibrated by suitable means, though the invention also comprehends vibrating the bed by vibrator devices inserted into or engaging the upper surface of the bed.

Vibration of the bed reduces the effects of friction therein, and between it and the confining box, so that the bed becomes more dense and uniform and is supported to a greater extent by floating on the molten metal with the consequence that its lower surface is depressed further into the bath to cause an increase in the wiping pressure.

Also, the lessening of the friction between the bed and the box enables a greater depth of bed to be effectively utilised to develop a desired wiping pressure. Thus, when a static wiping bed is initially vibrated, its upper surface is observed to fall until it reaches and remains substantially constant at a lower level.

It has also been found that the bed may be vibrated under such conditions that the bed forming material may be progressively discharged from the bottom of the bed at a restricted rate at which the bed remains substantially uniform and dense, the material thus discharged being replaced by the addition, either continuously or at suitable intervals, of fresh material to the top of the bed.

This progressive discharge or feed-out from the bottom of the bed has been found to be highly advantageous as it tends to maintain the bottom of the bed in a clean condition and so to contribute materially to the maintenance of substantially uniform coating weights, while obviating the usual operation of periodically manually scraping accumulated material from the bot tom of the bed.

Thus. because the bed, when vibrated, is supported to a greater extent by floating on the bath, its lower surface is depressed and by way of example, it has been found that, when using river gravel of specific gravity approximately 2.3, the bed may be substantially depressed in this way, e.g. by about 5 cms.

lf therefore the bottom of the wiping box is disposed at a lesser distance below bath level, c.g. about L 7 L875 cm, and the bath level is maintained at or near a constant height, the bottom layers of the bed can be induced to feedout from under the box and float to the surface. This is accompanied by a continuous gradual downward movement of the entire bed which is thus maintained in a constantly clean and mobile condition.

The preferred rate of discharge or feed-out for a gravel bed is in the range of about 25 to 50 mm of bed depth per hour, as this has been found sufficient to maintain the bed in a clean condition, though higher feed-out rates may be used without significant effect on the packing of the bed. However, in the case of gravel, feed-out rates substantially above mm per hour have, in some cases, been accompanied by a reduction in the degree of packing,

Experiments have shown that both bed compaction and feed-out may be achieved over wide ranges of frequencies and amplitudes. Thus, for gravel beds, advantageous results both in bed compaction and restricted feed-out have been achieved with frequencies in the range of 20-160 Hertz and small amplitudes in the range of 35-600 microns though the invention is not limited thereto.

However, for convenience of application and control and for consistency of results in a production unit, vibration levels with frequencies in the range 50l l0 Hertz and amplitudes of 35-150 microns are at present preferred.

No evidence has been found to indicate that the direction of vibration is critical though this does not preclude the possibility that for various reasons, some may be found more suitable than others in particular cases. At this stage, however, successful results have been obtained with uni-directional vibrators arranged vertically or laterally and also with centrifugal vibrators.

As hot dipped galvanising is at present most commonly used in the production of Type A coatings for which there is a larger demand than for the other coatings, work on this invention has so far been largely restricted to the production of Type A coatings though the invention is also applicable to the formation of coatings of both lighter and heavier weights, thus extending the application of hot dip galvanising to a wider range of coating weights particularly when used in conjunction with the gas wiping procedure.

The invention is further described below with reference to the accompanying drawings in which:

FIG. is a view in sectional end elevation of apparatus for wiping galvanised wire,

FIG. 2 is a view in front elevation,

FIG. 3 is a view in plan, and

FIG. 4 is a graph showing relationships between vibration levels and bed compaction and feed-out.

The illustrated apparatus comprises a wiping box or chamber, generally designated 10, which is rectangular in shape and which may conveniently be about 38 to 60 cms. in height, about 10 cms in width and of any re quired length.

This box, which is open both at the top and the bottom, and which extends across a bath 12 of molten zinc, with its lower end submerged therein, is supported in any convenient manner which permits of it being vibrated in a required manner, For this purpose, each end of the box may be secured to the free ends of canti lever arms 13 disposed approximately horizontally and secured at their opposite ends to a rigid support 130.

The box contains a wiping bed 14 of any suitable discrete material, preferably washed and graded river gravel, which floats on the molten zinc and when the apparatus is in use, any required number of spaced wires 16 are drawn vertically upwards through the wiping bed at a constant speed by a conventional take-up or winding frame which is not shown.

For this purpose, the wires pass from reels, not shown, to rollers 18 which are submerged in the zinc bath below the wiping box in the usual way.

The illustrated wiping box is so constructed that the aforesaid gas wiping method may be carried out by continuously supplying to the bottom of the bed, a suit able non-oxidising gas, e.g. propane, containing at least a small proportion of hydrogen sulphide or other suitable gas.

For this purpose, the back wall 101 of the box is formed with an internal cavity 20 which communicates mm 3.175 mm are shown graphically in FIG. 4.

In this Figure, the line A shows the relationships between frequency and amplitude at which compaction of the bed commenced while curve B shows the relationships at which feed-out commenced from the bottom of the bed. In the shaded area between A and B, therefore, the bed was in the desired compacted condition but feed-out did not take place.

It will be apparent from this graph that at very low frequencies or small amplitudes below the line A, there was no evident effect on the bed but at or about the frequencies and amplitudes represented by that line, a lowering of the surface of the bed becomes evident as the bed settles and becomes more dense. When the freat its lower end with the interior of the box by a horil5 quency and/or amplitude is further increased, little if zontal slot 21 which extends substantially the full any further change in the bed is observed until the length of the box. A horizontal tube 22 secured to the curve B is reached, when feed-out commences with back wall above the cavity communicates with the latconsequent progressive descent of the bed surface and ter by a longitudinal slot formed in the wall of the tube the appearance of discharged bed forming material on and at least one end of the latter is open so that it may 20 the surface of the zinc in the vicinity of the box. be connected to the gas supply. When the frequency and/or amplitude is or are in- However, while the wiping box is preferably so concreased still further, the feed-out rate increases until structed as to enable a gas to be directed into the inteen lly it m s 00 rapid f r practical purposes rior thereof, this is not essential for the purpose of this d t d m s leS dense. i n i 50 th t th b may b f any i bl 25 Within the shaded area of the graph therefore, there struction. is a wide range of frequencies and amplitudes which As shown in FIGS, 1 and 3, me n a r id d f will yield coatings significantly lower in weight and vibrating the box and for thi ur o e, any it bl imore uniform than are produced under similar condibrating means may be used. For example, and as shown tions without vibration and by adjusting frequency and in the drawings, a rotary vibrating device 30, driven by amplitude to a point at or above the curve B, it is possicompressed air, may be mounted on a br k t 32 atble to set up conditions where the benefit oflower coattached centrally to one side wall of the box, though varg Weight ca e Comb ned ith a rate of feed-out ious commercial uni-directional electro-magnetic vi- WhiCh ps e be Clean ithout depleting it too rapb m h l b d i h aqua] Success, idly. A satisfactory rate of feed-out is one which results Th b may b ib d verticany d or h i 35 in the lowering of the top of the bed at a rate of about tally in either its longitudinal or lateral direction or t0 P houh both and as yet, it has not been found that the direction An alternative Pmcedure has also been used and f ib i i i ifi B way f example, h i shown to be quite as effective. In this, continuous vibratal vibrations at 50 Hertz with an amplitude of about tion is pp to the Within the shaded area of the I50 microns in the longitudinal direction of the box 40 g p This level of Vibration Provides the have proved to be quite eff ti benefit of reduced coating weight but does not produce w (he wiping bed 14 whether composed f h the self cleaning action of automatic feed-out. Howcoal, grave] or other discrete material is subjected to ever hy sljlpenmposmg ahmtel'mmeht higher p vibration in this way, the granules pack more closely tude an amplitude Ofabout 1 mm), feedtogether and the level of the upper surface of the bed l' of gravel can be sfflected and the results is ]0Wered This lowering is due partly to the bed tamed are generally similar to those obtained when the coming more compact and partly to the f that the bed is vibrated under conditions represented by a point bottom of the bed descends further below the level of flPOve f the F DePsndlhg the ihdl the zinc bath as it is then supported almost entirely by dual mstananon s mlxed Vlbra "0h Procedure the latter whereas in the absence of vibration, it is supcolfld be more convemem than the first method ported to a greater extent by the friction between the scnbedgranules of the bed and between the latter and the wall h Q g ta e hows the results in respect of of um coating weights, obtained when several parcels of L4? In the application of the invention, several different produced by Gas w pmceSF conditions of the bed are produced at different vibral the W' g bed was Subjected to f cond" tion levels which in turn are different for different bed 9 vlbranon All other. plant condmons were forming materials, being related to factors such as the mammmed Constant and were shape, size, surface character and specific gravity of the material used. As an example of the stages relevantto fimgggfi sggfi gg this invention, certain results of work done using a wlp- Height of Gravel Bed 200 cm ing bed composed of washed and graded river gravels Thmughpu speed of specific gravity approximately 2.3, and size 6.35

TABLE I FRE4 AMPLITUDE COATING WElGHT g/m QUENCY HERTZ MICRON MAX. MIN. AVERAGE RANGE STD. DEVlATlON NIL NlL 552.3 292.9 384.5 259.4 54.9 20 800 332.6 207.5 277.7 125.1 27.5 32 360.1 231.9 308.2 128.2 35.7

TABLE l-Continued FRE AMPLITUDE COATING WEIGHT g/m" QUENCY HERTZ MTCRON MAX. MIN. AVERAGE RANGE STD. DEVIATION 44 150 375.3 350.2 295.) I251 27.5 55 120 320.4 250.2 292.9 70.2 15.5 72 100 350.9 222.8 289.9 l28.l 37.1

granules which float on said bath of molten zinc, and

The following table summarizes the results obtained h Several parcels f 2 4 mm i were; i fl fl b.v1brat1ng said bed to maintain it in a dense and uniproduced, the conditions, other than vibration levels, form Condition 50 that the coatings formed On the being constant, i.e.: wire or strip are more uniform in thickness.

2. The method according to claim 1 wherein the bed [5 is vibrated by continuously or intermittently vibrating Zmc TLfmpcmum 455C a box or other enclosure within which it is confined. lmmcrs1on Length 3.05 metres Heigh (3mm BEd 300 cm 3. The method accordmg to cla1m 1 wherein the bed Throughput Speed mlmm is vibrated under conditions such that bed forming material is progressively discharged from the bottom of TABLE II FRE- AMPLITUDE COATING WEIGHT g/rn QUENCY HERTZ MICRON MAX MIN. AVERAGE RANGE STD. DEVlATlON NIL NIL 595.0 396.6 491.3 193.4 32.9 NIL N11. 561.5 369.2 443.6 192.3 38.4 800 366.1 268.5 302.1 97.6 19.8 56 140 347.8 268.5 308.2 79.3 21.6 56 200 381.4 283.7 320.4 97.8 20.1 72 70 4024.9 386.8 341.7 122.1 27.7 72 160 4|8.U 303.2 369.2 l09.8 27.6 as 1011 427.2 329.5 375.3 97.9 23.1 1 10 50 390.5 302.1 350.9 88.4 23.4 166 50 482.1 335.6 408.9 146.5 25.6

it will be evident from each table that in each case, vibration of the bed under the specified conditions significantly reduced. the average coating weight, the range of maximum minimum coating weights and the standard deviation.

In conjunction with vibration of the bed as above described, a desired wiping pressure may also be achieved by selecting a material of a suitable specific gravity to form the wiping bed, or by loading the latter at the top such as by supporting small pieces of metal thereon.

For example, when using an unloaded bed composed of grave] of the aforesaid size range, it was found that coating weights throughout the range 240 to 490 grams of zinc per square metre of surface could be formed on wire of 3.42 mm diameter, by varying the throughput speed. When, however, the same bed was loaded at the top with pieces of /2. inch steel plate cut to size, the range of coating weights obtained, over the same range of wiping speeds was 180 to 430 grams per square metre which was sub-stantially the same as that obtained with an unloaded wiping bed, of the same depth, composed of zinc sinter (S.G.4.S

I claim:

1. The method of wiping hot dipped galvanized wire or strip comprising the steps of a. drawing said galvanized wire or strip upwardly from a bath of molten zinc through a laterally confined wiping bed of gravel or other hard and inert material in the form of free tumbling bodies or the bed at a restricted rate whereby the bed remains substantially dense and uniform, the material thus discharged being replaced by the addition, either continuously or at suitable intervals, of fresh material to the top of the bed.

4. The method according to claim 3, wherein the bed is continuously vibrated at a frequency and amplitude which suffice to maintain it in a substantially dense and uniform condition without causing discharge from the bottom thereof and is also intermittently subjected to vibrations of greater amplitude to effect discharge of the bed forming material.

5. The method according to claim 1 wherein the bed is composed of gravel and is vibrated within a frequency range of 20 l60 Hertz and an amplitude range of 35 600 microns.

6. The method according to claim I wherein the bed is composed of gravel and is vibrated within a frequency range of 50 1 l0 Hertz and an amplitude range of 35 microns.

7. The method according to claim 1 wherein the top of the bed is continuously pressed downwards by weights supported thereon or by a pressure plate actuated by springs or by fluid pressure.

8. The method as recited in claim I wherein vibration of said bed is continuous.

9. The method as recited in claim 1 wherein vibration of said bed is intermittent.

- UN I'IY-JD STATES PATENT c I-l-ICE CERTIFICATE OF C 03.11133 C'lI ON atcnt No. 3,892,894 PIltCd July l l9'Z5 ,6 Jack P. sciffe r [nvcrn L01 (3) It is certified that error appears in the above-identified patent 1nd that. said Letters Patent are hereby corrected as shown below:

Column 6, line 62, "Throughput Speed 86 l m/mir'1. should read throughput Speed 76.1 m/min.- ;j

Table II bridgingoolumns 7- and 8, line 1 ofthe data, "193.4" Juld read --l9.8.4'-,; line 5 offthe data, :"*97.8 should read 977; 1e 6 of the data, "386.8" should read -2 86 ..8-, line 8 of the data, 7.6" should read ,-.97 .7. 1 l

Signed and scaled this thirteenth Day of April1976 [SEAL] Arrest.

RUTH C. MASON C. MARSHALL DANN Arresting ()jficer ('umnu'ssium'r uj'larel'lls and Trademarks 

1. THE METHOD OF WIPING HOT DIPPED GALVANIZED WIRE OR STRIP COMPRISING THE STEPS OF A. DRAWING SAID GALVANIZED WIRE OR STRIP UPWARDLY FROM A BATH OF MOLTEN ZINC THROUGH A LATERALLY CONFINED WIPING BED OF GRAVEL OR TOHER HARD ANDINERT MATERIAL IN THE FORM OF FREE TUMBLING BODIES OR GRANULES WHICH FLOAT ON SAID BATH OF MOLTEN ZINC, AND B. VIBRATING SAID BED TO MAINTAIN IT IN A DENSE AND UNIFORM CONDITON SO THAT THE COATINGS FORMED ON THE WIRE OR STRIP ARE MORE UNIFORM IN THICKNESS.
 2. The method according to claim 1 wherein the bed is vibrated by continuously or intermittently vibrating a box or other enclosure within which it is confined.
 3. The method according to claim 1 wherein the bed is vibrated under conditions such that bed forming material is progressively discharged from the bottom of the bed at a restricted rate whereby the bed remains substantially dense and uniform, the material thus discharged being replaced by the addition, either continuously or at suitable intervals, of fresh material to the top of the bed.
 4. The method according to claim 3, wherein the bed is continuously vibrated at a frequency and amplitude which suffice to maintain it in a substantially dense and uniform condition without causing discharge from the bottom thereof and is also intermittently subjected to vibrations of greater amplitude to effect discharge of the bed forming material.
 5. The method according to claim 1 wherein the bed is composed of gravel and is vibrated within a frequency range of 20 - 160 Hertz and an amplitude range of 35 - 600 microns.
 6. The method according to claim 1 wherein the bed is composed of gravel and is vibrated within a frequency range of 50 - 110 Hertz and an amplitude range of 35 - 150 microns.
 7. The method according to claim 1 wherein the top of the bed is continuously pressed downwards by weights supported thereon or by a pressure plate actuated by springs or by fluid pressure.
 8. The method as recited in claim 1 wherein vibration of said bed is continuous.
 9. The method as recited in claim 1 wherein vibration of said bed is intermittent. 